This invention relates to electric vehicles and more particularly to safety features for use on an electric vehicle driven by a separately excited field control direct current drive motor.
In most prior art electric vehicle speed control system a direct current (DC) chopper control and a series wound drive motor are used. The chopper is placed in series between a DC power supply and series wound drive motor. The DC chopper controls the speed of the DC series motor by controlling and interrupting the full armature current. The speed of the DC drive motor is controlled by conventional armature chopper apparatus which provides complete control of the motor over its entire speed range. A prior art DC chopper controls the time of the pulses and/or the magnitude of the pulses being fed through the armature of the DC drive motor. By varying the parameters of the drive impulses being fed to the armature of the DC drive motor, the speed of the drive motor can be varied. DC chopper control for armature current coupled with a series wound drive motor is the most common approach used for driving prior art electric vehicles. Since the chopper controls the full armature current, it must necessity be of a large size to handle the large current involved. Prior art direct current choppers which are required in the armature circuit are bulky and expensive. The conventional chopper controller since it must interrupt and control full armature current requires expensive high-powered thyristors and complicated control circuitry for switching and controlling the current in the armature circuit. Due to the large currents being interrupted, cooling of the controller is a problem. Conventional chopper controllers for armature current are also rather inefficient at low to normal operating speed.
An electronics system for controlling the torque speed characteristic and regenerative braking of a separately excited drive motor for use on electric vehicles is disclosed in cross-reference U.S. patent application Ser. No. 346,552. The torque speed characteristics can be controlled by varying the separately excited shunt field of the direct current drive motor. As disclosed in the above-mentioned application, speed of the direct current drive motor is controlled from some base speed to a maximum speed.
Safety and protection features are very important in the control of an electric vehicle. It is desirable that the safety features provided operate without conscious action by the driver to protect the vehicle from dangerous operating conditions. It is desirable to protect the vehicle and motor from overspeed, heavy overcurrent which could lead to motor damage or a possible fire, excessive field currents which could damage the motor, a stuck throttle which could cause a runaway condition, drive motor overheating which could damage the motor, and loss of field current. It is desirable that the safety features when required operate so as to reduce the operating performance of the electric vehicle while allowing continued operation of the vehicle, rather than complete shutdown of vehicle operation.